Steam cracking of hydrocarbons has typically been effected by supplying the feedstock in vaporized or substantially vaporized form, in admixture with substantial amounts of steam, to suitable coils in a cracking furnace. It is conventional to pass the reaction mixture through a number of parallel coils or tubes which pass through a convection section of the cracking furnace wherein hot combustion gases raise the temperature of the reaction mixture. Each coil or tube then passes through a radiant section of the cracking furnace wherein a mulitplicity of burners supply the heat necessary to bring the reactants to the desired reaction temperature and effect the desired reaction.
Of primary concern in all steam cracking processes is the formation of coke. When hydrocarbon feedstocks are subjected to the heating conditions prevalent in a steam cracking furnace, coke deposits tend to form on the inner walls of the tubular members forming the cracking coils. Not only do such coke deposits interfere with heat flow through the tube walls into the stream of reactants, but also with the flow of the reaction mixture due to tube blockage.
At one time, it was thought that a thin film of hydrocarbons sliding along the inside walls of the reaction tubes was primarily responsible for coke formation. According to this theory, a big part of the temperature drop between the tube wall and the reaction temperature in the bulk of the hydrocarbon process fluid takes place across this film. Accordingly, an increase in heat flux, meaning a rise in tube-wall temperature, called for a corresponding increase in film temperature to points high enough to cause the film to form coke. Thus, coke was thought to be prevented by using lower tube-wall temperatures, meaning less heat flux into the reaction mixture and longer residence times for the reactions.
In order to achieve high furnance capacity, the reaction tubes were relatively large, e.g., three to five inch inside diameters. However, a relatively long, fired reaction tube, e.g., 150 to 400 feet, was required to heat the fluid mass within these large tubes to the required temperature, and furnaces, accordingly, required coiled or serpentine tubes to fit within the confines of a reasonably sized radiant section. The problems of coke formation, as well as, pressure drop were increased by the multiple turns of these coiled tubes. Also, maintenance and construction costs for such tubes were relatively high as compared, for example, with straight tubes.
In a 1965 article, entitled "ETHYLENE", which appeared in the November 13 issue of CHEMICAL WEEK, some basic discoveries that revolutionized steam cracking furnace design are disclosed. As a result of these discoveries, new design parameters evolved that are still in use today.
As disclosed in the article, researchers discovered that secondary reactions in the reacting gases, not in the film, are responsible for tube-wall coke. However, shorter residence time with more heat favors primary olefin-forming reactions, not these secondary coke-causing reactions. Accordingly, higher heat flux and higher tube-wall temperatures emerged as the answer.
The article also indicates, however, that reduced residence time is not a simple matter of speedup (of flow of process gas through the tubes), as the heat consumed by cracking hydrocarbons is fairly constant--about 5,100 BTU/lb. of ethylene. Consequently, it suggests that a shorter residence time requires that heat must be put into the hydrocarbons faster. Two feasible ways suggested for expanding this heat input are by altering the mechanical design of the tubes so they have greater external surface per internal volume and increasing the rate of heat flux through the tube walls. The ratio of external tube surface to internal volume, it is disclosed, can be increased by reducing tube diameter. The rate of heat flux through the tube walls is accomplished by heating the tubes to higher temperatures.
Thus, the optimum way of improving selectivity to ethylene was found to be by reducing coil volume while maintaining the heat transfer surface area. This was accomplished by replacing large diameter, serpentine coils with a multiplicity of smaller diameter tubes having a greater surface-to-volume ratio than the large diameter tubes. The coking and pressure drop problems mentioned above were effectively overcome by using once-through (single-pass) tubes in parallel such that the process fluid flowed in a once-through fashion through the radiant box, either from arch to floor or floor to arch. The tubes typically have inside diameters up to about 2 inches, generally from about 1 to 2 inches. Tube lengths can be about 15 to 50 feet, with about 20-40 feet being more likely .
Accordingly, it is most desirable to utilize small diameter (less than about 2 inch inside diameters), once-through reaction tubes with short residence times (about 0.05 to 0.15 seconds) and high outlet temperatures (heated to about 1450.degree. F. to 1700.degree. F.), such as disclosed in U.S. Pat. No. 3,671,198 to Wallace. But while this reference typifies some of the key advantages related to state-of-the-art furnace technology, it also typifies some of the serious disadvantages related to the same.
During operation of the furnace, the tremendous amount of heat generated in the radiant section by the burners will cause the tubes to expand, that is, experience thermal growth. Due to variations in process fluid flow to each tube, uneven coking rates, and non-uniform heat distribution thereto from the burners, the tubes will grow at different rates. However, since the coil is now formed from a multiplicity of parallel, small diameter tubes fed from a common inlet manifold and the reaction effluent from the radiant section is either collected in a common outlet manifold or routed directly to a transfer line exchanger, the tubes are constrained. That is, there is no provision to absorb the differential thermal growth amongst the individual tubes. The thermal stresses caused by differential thermal growth of the individual tubes can be excessive and can easily rupture welds and/or severely distort the coil.
As shown in Wallace, this differential thermal growth is typically absorbed by providing each tube with a flexible support comprised of support cables strung over pulleys and held by counterweights. Each flexible support must absorb the entire amount of thermal growth experienced by its corresponding reaction tube, typically as much as about 6 to 9 inches, and is also used to support the tube in its vertical position. This flexible support system also makes use of flexible-tube interconnections between the inlet manifold and the reaction tubes to absorb differential thermal growth thereof as shown, for example, in FIG. 2 of Wallace. This flexible-tube interconnection typically takes the form of a long (up to about 10 feet) flexible loop, known as a "pigtail", of small diameter (about 1 inch) located externally to the radiant section. The pigtail has a high pressure drop and, therefore, cannot be used at the outlets of the reaction tubes as one of the objectives in operating the furnace is to reduce pressure drop.
When used at the inlets to the reaction tubes, these pigtails can interfere significantly with critical burner arrangements. One of the major constraints limiting the reduction in residence time and pressure drop is the allowable tube metal temperature. In order to keep tube metal temperatures within acceptable ranges for current day metallurgy, it is desirable to arrange the flow of reaction fluid so that the lowest process fluid temperatures occur where the burner heat release is highest. This requires locating burners at the inlet of the coil, i.e., for process fluid flow from floor to arch (ceiling), burners are located at the floor and for process fluid flow from arch to floor, at the arch. It is, thus, undesirable to locate the pigtails at the coil inlet because they interfere with access to the furnace for maintenance or process change purposes. For example, it is periodically necessary to pull burners for routine maintenance or replacement. Also for example, it may be desirable to modify the burners so as to provide for air preheat thereto. With the pigtails in the way, these tasks become increasingly difficult and burdensome.
Because the pigtails are made of flexible material incapable of structurally supporting the radiant tubes, separate support for the tubes is required, adding to the overall expense for the furnace. Also, the use of long, small diameter tubing at temperatures at which small amounts of coking occurs increases the chances for experiencing coking problems. Should such problems occur, the pigtails can be so difficult to clean-out that they most likely will require cutting out in order to remove the coke from the furnace system. Furthermore, the pigtails are made of material that is highly susceptible to cracking from the extreme heat generated by the steam cracking process, potentially requiring frequent replacement.